Metallurgical coke suitable for use in the very large blast furnaces being built today must have very high coke strength as indicated by hardness and stability. A minimum hardness of about 68 and a minimum stability of about 55 are generally essential. "Stability" as defined herein is the strength of the coke to withstand breakage as given by the ASTM Stability Index test and "hardness" as defined herein is the measure of coke hardness indicated by the ASTM Hardness Index test, according to ASTM Procedure D3402. To achieve this quality of coke requires the use of expensive high quality coking coals having a high percentage of volatiles in the coal, a high fluidity and a relatively low percentage of inert components. Due to the desire to use less expensive coals and also due to the decreasing supply of the high quality metallurgical grade coals great effort has been expended upon ways of achieving high strength coke using lower quality coals. See "The Critical Case of Coke", Journal of Metals, February 1972, pp 32-34, incorporated herein by reference.
The Japanese, for example, have conducted several studies on how to combat the decrease in coke quality due to the use of inferior coals. Controlling the size of the coal being charged to up to about 90% below 3 mm diameter plus adding oil to remedy the lower bulk density resulting from the harder crushing has been developed by Nippon Steel. This process is very expensive due to the high cost of oil. Another technique also developed by this same company involves mixing coal briquettes with charging coal. This technique (1) has limitations on the bulk density of the charge due to difficulties in pushing, (2) requires increased surveillance of temperature control and distribution in the coking chambers, and (3) has problems with segregation of briquettes in the charge.
Formcoke is another approach being pursued but due to the high cost of such coke it is believed to be only a supplementary source for coke requirements.
Preheating of the finely ground coal charged to the coke ovens is currently used commercially to achieve high coke strength from lower quality coals. However, in addition to being costly due to the energy demands for heat and capital costs this process is very dusty. This dustiness besides being a pollution problem results in coal dust getting mixed with the pitch and other liquid hydrocarbons produced as a by-product from the coke ovens, thus destroying the value of these hydrocarbons for certain chemical uses, such as for making electrodes.
A method of coal compaction which is currently used for coke making in certain parts of the world is "stamp charging". In this method the entire quantity of coal is first compacted into one big block. The ovens are then opened and the solid block is pushed into the oven. Since the oven must be open during the charging, there are very serious pollution problems due to the dust and fumes given off during the charging. See N. N. Das Gupta, The Use of Tall Ovens and Stamp Charging for Coking Indian Coals, Journal of Mines, Metals, and Fuels, Vol. 14, No. 8, August, 1966, pp 256-263; and Coke and Chemistry, U.S.S.R., No. 12, 1960, pp 54-58; each incorporated herein by reference.
A process for producing coke comprising briquetting finely divided coal followed by increasing the bulk density prior to the carbonization step by filling the free spaces between the briquette with chippings is taught in Henry Zielinksi, "Present Method of Coke Manufacture", (1972) pp 58-62, incorporated herein by reference. However, this process requires the briquette chippings be screened off and returned for briquetting again. This screening step is an additional expense as well as cutting down on the rate of throughput of coal through the process, thus decreasing the efficiency of the process. The screening step also aggrevates the already serious air pollution problem caused by dustiness from the briquettes.